I can tell you that for me the smallest amount of caffein would cause my breats to be super tender. I have had cysts for over twenty years. I don't know what "regular" soda would be caffein free unless it says "caffein free". Even decaf coffee gas 3% caffein in it, so I can have problems on three cups of decaf. When you reach menopause they say the cysts shrink, however I hit menopause seven years ago and they have not left my breats yet unfortunately. Good luck.
In Fibrocystic Breast Condition is effected by an increased amount of Estrogen within the body ... hence the increased breast tenderness that fluctuates with the menstrual cycle. Caffeine increases the amount of acid in the body and interferes with the acid-base balance thus causing the body to produce more Estrogen than Progesterone. It's not only coffee ... tea, high energy drinks and chocolate are also caffeine rich. Most regular soda today is caffeine free.